U0401: OBD-II code U0401 flags that another module received invalid or implausible data from the ECM/PCM — the bus works, but the message payload is wrong.

Symptoms

• Check engine light is on and possibly ABS or traction lamps as well; the vehicle may run oddly.
• Transmission shifts, stability interventions, or torque control may behave incorrectly because they act on bad data.
• Multiple companion codes usually appear alongside — the invalid data affects any module reading PCM outputs.

Common causes

• A failing PCM broadcasting bad values — usually paired with internal PCM codes like P0606.
• A software mismatch between modules after a partial reflash or replacement without proper configuration.
• Rarely, EMI/RFI corrupting the bus data in transit — usually only after aftermarket wiring modifications.

What to do?

Scan the PCM for its own codes first — U0401 is usually a downstream symptom of another PCM fault. Fix PCM internal faults, then clear codes. Verify all modules are on the correct software version for the vehicle. Reprogram or replace the PCM if bad values continue.